The present invention relates to electronic apparatus such as printed circuit boards. The invention relates more specifically to electronic apparatus such as printed circuit boards that provide isolation among multiple ground conductors having different ground potential.
Modern electronic equipment commonly operates using active signals that are generated with reference to a ground potential. However, for a variety of physical and environmental reasons, different electronic systems may operate with different ground potentials. Shielded cables are widely used to conduct and carry electrical signals from one grounded device or system to another. Typically, a shielded cable comprises one or more signal conductors, usually comprising insulated wire, surrounded by one or more shields or ground conductors. The ground conductors are electrically isolated from the signal conductors by a dielectric layer.
Interconnecting systems having different grounding potentials can present a safety hazard when the interconnection is done using shielded cables. The cable shield provides a direct current (DC) electrical interconnection, and can therefore allow currents to flow on the shield, creating a safety hazard. However, shielded cable assemblies do provide a mechanism for reducing radio-frequency (RF) emissions from electronic equipment, and may be required for the systems to meet laws and regulatory standards governing generation of RF emissions or interference by such equipment.
One approach for both achieving safety and meeting RF emission requirements is to AC ground the shield to the system. This provides a high frequency ground path, resulting in reduction of RF emissions, and provides a low frequency DC open, thereby eliminating the flow of DC currents on the shield. However, this approach is undesirable, because it requires capacitors to achieve AC termination. In many circuit board structures, use of such capacitors occupies valuable circuit board space and reduces the maximum number of interconnections that can be fitted into a given area of a board (xe2x80x9cinterconnection densityxe2x80x9d).
A specific problem that arises in this context relates to deployment of E1/E3 metallic telecommunications equipment in the United Kingdom. Such equipment requires the use of a type 42 coaxial connector. In the connector, the coax shield for the transmit signal is DC grounded, while the coax shield for the receive signal is AC grounded. Providing proper AC grounding requires the use of capacitance referenced to the shield and the system ground. To provide the capacitance, one or more capacitors must be placed on the circuit board where connectors join the board. This consumes space; a reduction in the utilized space would increase port density within a given space.
Based on the foregoing, there is a clear need in the field of electronics for a method and apparatus that provides electrically safe termination of a plurality of ground sources that have different ground potentials.
There is a specific need for a method and apparatus that provides such termination, and that maximizes theoretical interconnection density.
The foregoing problems and objectives, and other problems and objectives that will become apparent from the following description, are addressed by the present invention, which comprises, in one aspect, a circuit board for terminating shielded conductors having multiple different ground potential values. In a signal interconnection region, a multi-layer laminated circuit board comprises a shield termination layer that is capacitively coupled to a separate ground reference layer by a dielectric layer. A signal layer is electrically isolated from both the ground reference layer and shield termination layer by a substrate. A shield of a shielded cable terminates at the shield termination layer of the interconnect region, and a signal conductor of the cable terminates at the signal layer. Buried capacitance is developed between the shield termination layer and the ground reference layer. As a result, RF emissions are minimized and an electrically safe termination of multiple ground sources is provided, without enlarging the surface area of an interconnection. In the specific context of E1/E3 equipment, buried capacitance may be incorporated into the back plane printed circuit board of a unit of telecommunications equipment. As a result, port density may be increased, while maintaining simplicity. The value of the capacitance can be varied by increasing or decreasing the physical area of the plane referenced to the shield of the cable through the connector.
In one aspect, the invention provides a circuit board, comprising a signal conducting layer adapted for electrically coupling to a signal conductor of a shielded cable; a ground reference layer that is electrically isolated from the signal conducting layer by a substrate; and a shield termination layer that is separated from and capacitively coupled to the ground reference layer by a dielectric layer that is interposed between the ground reference layer and the shield termination layer, and adapted for coupling to a shield of the cable.
One feature of this aspect is a connector having a shield element that is electrically coupled to the shield termination layer and a signal conducting element that is electrically coupled to the signal conducting layer. According to another feature, the ground reference layer is electrically coupled to a ground reference having a different electrical potential than a shield potential that is conducted by the shield reference layer.
In another aspect, a circuit board comprises a signal conducting layer adapted for electrically coupling to a signal conductor of a shielded cable; a plurality of ground reference layers that are electrically isolated from the signal conducting layer by at least one substrate layer; and a plurality of shield termination layers that are interleaved with, separated from and capacitively coupled to the ground reference layers by a plurality of interposed dielectric layers, wherein at least one of the shield termination layers is adapted for coupling to a shield of the cable.
In another aspect, a circuit board has one or more interconnect regions that receive a Type 42 coaxial cable connector, each of the interconnect regions comprising a signal conducting layer adapted for electrically coupling to a signal conductor of a shielded cable that terminates in the connector; a ground reference layer that is electrically isolated from the signal conducting layer by a substrate; a shield termination layer that is separated from and capacitively coupled to the ground reference layer by a dielectric layer, and adapted for coupling to a shield of the cable.
One feature of this aspect comprises at least two (2) interconnect regions consisting of a first interconnect region that receives a transmit connection and a second interconnect region that receives a receive connection, wherein a first shield conductor associated with the transmit connection is grounded to the ground reference layer, and wherein a second shield conductor associated with the receive connection is grounded to the shield termination layer.
Advantageously, the time required to manufacture a circuit board is reduced. Further, a reduction in the amount of time needed to set up a manufacturing line for a particular product may be reduced, thereby improving overall manufacturing efficiency.
Other aspects will become apparent from the appended specification and claims.